Ultrastructural demonstration of noradrenergic synapses in the rat central nervous system by dopamine-beta-hydroxylase immunocytochemistry.

Noradrenergic (NA) cell bodies and axonal processes were identified in the electron microscope by the immunocytochemical localization of the norepinephrine-synthesizing enzyme, dopamine-beta-hydroxylase (DBH). DBH immunoreactivity, visualized by the peroxidase-antiperoxidase method, was observed in the somata and proximal processes of locus coeruleus neurons and in the distal axons of several NA terminal fields. DBH immunoreactivity is distributed throughout the cytoplasm of the NA neuron, but demonstrates a selective association with endoplasmic reticulum, Golgi apparatus, mitochondrial outer membranes, large granular vesicles, and small, round synaptic vesicles. DBH-positive axonal profiles, typically interspersed between unlabeled dendrites, form two distinct populations: a) thin, unmyelinated intervaricose segments (ca. 0.28 micron) and b) spherical varicosities (ca. 1.00 micron). No DBH-positive varicosities were observed in contact with intracerebral capillaries. In order to determine whether or not NA axons typically form synaptic contacts, a quantitative analysis of selected areas of the diencephalon, cerebellum, and limbic cortex was carried out. More than half (58%) of all DBH-positive varicosities form axodendritic synapses characterized by specialized junctional appositions. The results suggest that NA neurons typically exert their influence on other neurons through highly restricted synaptic contacts.